Method and means for combining frequencies



Aug. 20, 1929. o PETERSON 1,725,721

METHOD AND MEANS FOR comsmms FREQUENCIES Filed May 23, 1927 lmmm lNVENTOR HAROLD O. PETERSON 12 2944 (fit M 4 AT ORNEY Patented Aug. 20, 1929.

UNITED STATES PATENT OFFICE.

HAROLD O. PETERSON, 0F RIVERHEAD, NEW YORK, ASSIGNOR '10 RADIO CORPORA- TION OF AMERICA, A CORPORATION OF DELAWARE.

METHOD AND MEANS FOR COMBINING FREQUENCIES.

Application filed May 23,

This invention relates to a method and means for combining frequencies, and more particularly for combining signal energy with energy from a local oscillator for heterodyning.

If signal energy and energy from a local oscillator are applied in the usual manner across a combining circuit it is found that the local oscillator tends to radiate energy through the signal circuit. This is disadvantageous because of the effect upon other receivers as well as the waste of energy and it is an object of my invention to prevent such reradiation.

The type of local oscillator usually employed in superheterodyne and heterodyne receivers employs a vacuum tube and associated tuned circuits, which are susceptible to frequency changes, and which therefore are usually carefully shielded from the other receiver circuits. Even then there is undesirable inter-action between the receiver and the oscillator, so that tuning of the receiver circuits ahead of the combining tube affects the tuning of the oscillator and vice versa. Accordingly it is a further object of my invention to so couple circuits resonant at different frequencies to a combining unit that the circuits will be independent in tuning.

These objects I accomplish by coupling a signal circuit to the input reactance of a combining unit, applying local oscillator energy to the center of the reactance, and causing it to flow equally and oppositely through the halves of the reactance so'that its effect on the signalling circuit is neutralized. From another aspect it consists in forming a re actance bridge, coupling the circuits for the two frequencies which are to be combined across different pairs of opposite corners of the bridge, symmetrically, and coupling the wave combining unit to the bridge unsynr metrically.

The invention is described more in detail in the following specification accompanied by a drawing in which Figure 1 is a wiring diagram forone form of my invention; and

Figure 2 is a portion of Figure 1 rearranged in the form of a bridge.

Referring to Figure 1, the .signal circuit includes an antenna 2, a tuning condenser 4, a coupling coil 5, and a grounded connection 8. Within a double copper shield 10 there is mounted a local oscillator 12, the frequency 1927. Serial No. 193,411.

of which is adjusted by means of a variable condenser 14. The wave combining unit, or rectifying unit, consists of a triode or three electrode vacuum tube 16 having a cathode 18, a grid or control electrode 20, and a plate or anode 22. The input circuit comprises a two part coupling reactance, exemplified in this case by the inductances 24 and 26, and a variable tuning condenser .28. One side of this resonant input circuit is connected to the control electrode 20 while the other side is connected to the cathode 18 through the adjustable series condenser 30.

Energy from the local oscillator. is conveyed to the combining unit through an oscillator circuit 32, which is coupled to the oscillator at 34, as shown, and to the coupling reactance between the coils 24 and 26. The oscillator circuit is completed by way of the grounded shielding through a grid biasing battery, 36, which makes the tube 16 operate at an asymmetric portion of its characteristic curve, to obtain rectification. There is an output circuit 86 connected between the anode 22 and the cathode 18 to deliver heat energy of intermediate frequency to a coil 38 leading to an intermediate frequency amplifier 40, which is followed by a receiver in customary manner.

In the simplest form of my invention the coils 24 and 26 are equal, and the condenser 30 is equal to the natural overall grid to cathode capacitance, that is, the capacitance to filament, shielding, and ground. The operation may be explained as follows: Signal energy applied to the input circuit 24, 26, 28, which is tuned to the signal frequency, causes signal current to flow thru the two parts of the coupling reactance 24 and 26 in series and the potential fluctuation between the terminal points A and B is applied to the control electrode 20. Oscillator energy from the circuit 32 flows thru the two parts of the coupling reactance in parallel, that is, it divides and flows equally and oppositely in the coils 24 and 26, in consequence of which it .is not transferred through the coil 6 to the antenna 2. The terminals A and B fluctuate equally and simultaneously in potential, and if the terminal B were connected directly to the cathode 18 the oscillator energy would have no effect, for the grid and cathode would be at the same potential. But according to my invention thecondenser 30 is inserted between the point B and the cathode 18, and this is of very small magnitude and therefore of high reactance, and the potential fluctuations at B are overcomethrough this reactance before reaching the cathode 18. The control electrode has impressed upon it both signal and local oscillations, and it being suit ably biased by the battery 36, the oscillations are combined, and their resultant beat is delivered to further circuits.

The operation of the circuit may be perhaps more exactly explained in connection with Figure 2, which shows a portion of the wiring in Figure l rearranged in the form of a bridge. In this figure it will be seen that the bridge is composed of the input coupling coils 24,26, the condenser 30, and the natural overall grid to filament capacitance C. Signal energy in the antenna circuit 2, 4:, 6, 8 is, in effect, applied to the bridge at the points A and B, while energy from the local oscillator 12 is applied through the circuit 32 to the other corners of the bridge.

Since the oscillator and the signal circuits are connected in a symmetrical manner to different pairs of opposite corners of a balanced bridge they have no effect upon one another either as regards reradiation or tuning. But

since the combining unit is connected unsymmetrically in the 'bridge, both the signal and the oscillator energies cause potential differences to be applied thereto.

The bridge is most simply balanced when the coil 2& equals the coil 26, and the capacitance 0 equals that of the condenser 30, but it is clear that the coils 24, 26 need not be equal, for if unequal it is merely necessary to adjust the variable condenser 30 to bring the bridge to balance. Thus, if the coil 26 is smaller than coil 24:, the condenser 30 should be made larger, thereby sending more current thru the coil 26 to obtain equal magnetic ef fects. From another viewpoint the arms 26 and 30 are equally decreased in reactance, as a result of which the terminal point B will remain at the same potential as the terminal point A with respect to the oscillator energy, which is the condition to be fulfilled to pre vent back coupling to the coil 6. The method of the invention may therefore be described as applying signal energy across the terminals of an input reactance of a combining unit, applying local oscillator energy to the reactance at a point intermediate the terminals in potential, and causing the current flow from the local oscillator in both parts of the reactance to be of such relative magnitude that the potential fluctuations caused by the oscillator at the terminals of the reactance are equal and in phase.

The reactances 24 and 26 need not be inductances, for capacity coupling is equally feasible. The combining unit or device of asymmetric characteristic employed need not necessarily be a vacuum tube, for other known means such as a saturable iron cored impedance may be used, and if a vacuum tube is used, it need not necessary be given its asymmetric characteristic by means of a biasing battery, although I think the arrangement shown is a preferable one. The output frequency may be an intermediate frequency which must subsequently be detected to ob tain an audio frequency, or it may, if desired, be an audible frequency directly after heterodyning as in the case of telegraphy signals.

I claim:

1. In a heterodyne receiver, a combining tube having a cathode and a control electrode, a circuit including a two part coupling reactance and a series condenser connected be tween said electrodes, a signal circuit coupled to the two parts of the coupling reactance in series, and a local oscillator coupled to the two parts of the coupling reactance in parallel, the series condenser being of proper magnitude to cause the oscillator currents to so divide in the parts of the coupling reactance that they neutralize one another with respect to the signal circuit.

2. In a heterodyne receiver, a combining tube having a grid and a grounded cathode, an input circuit comprising a coupling inductance and a tuning condenser connected in parallel therewith, means connecting one side of the input circuit to the grid, means including a series condenser whose capacitance equals the grid to ground-capacitance connecting the other side of the input circuit to the cathode, and a local oscillator circuit connected between the cathode and the midpoint of the coupling inductance.

3. An arrangement for combining energies of different frequencies obtained from differently tuned circuits while maintaining independence of tuning in said circuits comprising a combining tube having a grid and a cathode, a symmetrical bridge of reactances one arm of which is the natural overall grid to cathode impedance, a circuit for supplying energy of one frequency coupled across opposite corners of the bridge, a circuit for supplying energy of another frequency coupled across the other corners of the bridge, and an output circuit.

4;. In a heterodyne receiver,a three electrode vacuum tube having a grid and a grounded cathode, a reactance bridge, coupling coils comprising two adjacent arms of the bridge, a condenser and the natural grid to ground capacitance forming the other two arms of the bridge, a signalcircuit coupled across the bridge at the ends of the coupling coils, a local oscillator circuit connected between the coupling coils and across the other corners of the bridge, means to make the tube rectify, and an output circuit.

HAROLD O. PETERSON. 

